It is generally known to use passive safety devices in motor vehicles for protecting the passengers against injuries in the event of an accidental crash or impact. Such passive safety devices include, for example, inflatable airbags and devices for tightening safety belts. It is a special goal in the art to install a triggering apparatus together with a passive safety device, such as an airbag, in the small space of a steering wheel of a motor vehicle.
German Patent Publication No.: (DE-OS) 2,222,038 discloses an apparatus for triggering passive safety devices of the above described type comprising an acceleration sensor, preferably made of a piezoelectric ceramic material. The sensor output signals exceeding a certain minimum threshold value, are integrated in an integrator circuit. If the integrated signal exceeds another threshold value provided by a threshold value circuit or threshold value switch, then the safety device is triggered. Additionally, measures must be taken to ensure that the safety device is only triggered in the case of an actual accidental crash or impact, but is not triggered in other cases, despite the generation of significant signals by the acceleration sensor. The latter case arises, for example, when a short duration, but high mechanical load acts on the acceleration sensor, but will not cause any injuries to the occupants of the vehicle because of the short duration despite its size. Such short duration accelerations can arise, for example, if the vehicle is driven over a roadside curb at a higher speed, or if the acceleration sensor senses the impact of a stone or other road debris, or by a hammer blow while the vehicle is being repaired in a workshop or even by a strong blow of a driver's hand in the case of an installation in a steering wheel. In the known apparatus described in German Patent Publication (DE-OS) 2,222,038, triggering of the passive safety device in such cases is prevented because the output signals of the acceleration sensor are amplified and filtered and then nonsymmetrically limited in their amplitude, whereby the limit value for the positive amplitude is higher than that for the negative amplitude. In this manner it is achieved that during a short duration impact of the acceleration sensor, its integrated output signal will not reach the prescribed threshold value.
Furthermore, in the known apparatus the limited output signals of the acceleration sensor are supplied to a second threshold value switch of which the output is connected to the input of an AND-gate having a second input connected to the above mentioned threshold value switch connected to the output of the integrator. The mentioned AND-gate thereby forms a logic circuit which controls the safety device which is thus triggered only if the integrated signals exceed the first threshold value and simultaneously the acceleration still remains effective on the acceleration sensor. Especially when a triggering apparatus and an airbag are installed in a steering wheel the triggering criteria for the safety device must be very precisely differentiated because the signals from an acceleration sensor mounted in the steering wheel do not reach as significant a level as those from acceleration sensors alternatively mounted in other locations of the vehicle which are more subject to direct impact forces in the case of a crash.
Other triggering devices are known which use several acceleration sensors and evaluate the signals of the acceleration sensors in several evaluation channels according to several different respective evaluation criteria, for example, for distinguishing a rear impact from a front impact or a side impact, or even a hammer blow on one of the sensors as disclosed in the above mentioned copending application U.S. Ser. No. 07/352,928. The electronic circuitry in the copending triggering apparatus is sophisticated enough to take a plurality of criteria into account and may be specially constructed to take into account special vehicle characteristics. In such apparatus the acceleration sensors are located at various significant locations in a vehicle, which locations are directly susceptible to impact in the case of a collision. However, an arrangement of several acceleration sensors in the steering wheel of a vehicle cannot achieve such advantages because the forces acting on the sensors in the case of a collision or impact, are not as significantly differentiated from one another as in the case of several impact sensors arranged at separate locations in the vehicle.